DSLR cameras explained: 10 things to know about single lens reflex

For nearly decades, both professional and amateur photographers alike have preferred the single lens reflex camera. But what is a DSLR camera, how do they work, and why are they so popular? In this tutorial we explain the key points you need to know when making the jump to shooting with a DSLR camera.

1. What is Single Lens Reflex?

While the name ‘single lens reflex’ camera seems a bit archaic today, there was a time when twin lens reflex (TLR) cameras were in high demand – and there are still a couple of models on sale today.

The two lenses of a TLR have the same focal length and their focusing mechanisms are linked, but they are used for two distinct tasks. The ‘viewing lens’ is intended for focusing while the photographer looks in the waist-level viewfinder, while the ‘taking lens’ sits in front of the film, ready for exposure in a different chamber.

The word ‘reflex’ comes from the fact that TLR and DSLR (DSLR simply means ‘digital single lens reflex’) cameras both have a reflex mirror – basically a mirror at 45 degrees – that reflects light from the lens into the viewfinder.

In a TLR, the mirror is fixed and the scene is visible in the viewfinder throughout the exposure.

In a DSLR camera, however, the mirror will flip up during exposure in order to permit the light to reach the film or sensor. This blanks out the viewfinder for the length of the exposure.

2. The viewfinder is optical

DSLRs have an optical viewfinder that collects light from the same lens that is used to capture the image. The light that is exiting the lens is reflected up into a pentaprism (or pentamirror) when the mirror is down.

The pentaprism then bounces the light around to produce an image on the viewfinder screen that is the right way round. This allows SLRs to be smaller in size than TLRs and also resolved the issue of parallax error met with rangefinder cameras – they do not perceive the scene through the lens.

Contemporary SLR viewfinders are typically bright and user-friendly, but unlike the electronic viewfinders found in several compact system cameras, they cannot display the impact of camera settings.

Just prior to exposure, the reflex mirror lifts in order to permit light to reach the sensor. This in turn makes the viewfinder go black during exposure.

3. Exposure is controlled by shutter speed, aperture and sensitivity

With a DSLR camera, there are three means of managing exposure; shutter speed (exposure time), aperture, and sensitivity.

Shutter speed and aperture size determine the amount of light that reaches the sensor, but the camera actually has a constant base sensitivity that consistently remains.

Other sensitivity settings are made by applying gain (amplification) to the image signal (more on this point later) to reproduce the effect of a more sensitive medium.

4. Most DSLR cameras offer a collection of exposure modes

The majority of modern DSLR cameras provide a wide range of exposure modes from completely manual to completely automatic options, such as aperture priority and shutter priority mode in between.

Typically, there is also a collection of scene modes in which the camera selects exposure and processing settings that are fitting to that particular type of scene.

These scene modes may be chosen by the photographer, but several cameras now provide a mode in which the camera pre-empts the photographer by detecting the type of scene and then applying appropriate exposure and processing settings.

In aperture priority mode, the photographer sets the aperture to control depth of field and the camera sets a shutter speed that will produce a good exposure.

On the other hand, in shutter priority mode the photographer sets the shutter speed to freeze or blur movement according to their needs, and the camera sets a proportionate aperture. The photographer possesses complete control over the shutter speed and aperture in manual mode.

Several DSLR cameras now offer photographers the choice to set sensitivity automatically. The photographer still sets the shutter speed and aperture if this mode is selected when shooting in manual exposure mode, but the camera establishes the overall exposure by varying sensitivity as it calculates is required from shot to shot.

5. The sensor produces an electrical signal

Once the reflex mirror is lifted and the shutter opened, the sensor is exposed to light allowing an image to be recorded. The sensor is covered with photo receptors, frequently referred to as pixels, each of which has a micro lens over it to guide the light downwards.

Typically the sensor has a Bayer pattern filter array over it, this allows the camera to interpret colours even though each receptor actually only detects luminance (brightness).

This red, green, green, blue (RGGB) filter has two green filters for every red or blue filter to reflect the dominance that green has in our vision.

Light falling on the sensor produces an electronic signal. The brighter the light is, the stronger the signal will be. This electronic signal is then converted into a digital signal, which can then be processed into a digital image.

6. They have two AF systems

There is a dedicated autofocus (AF) sensor which is used to focus the lens when the viewfinder is used for composing images. This system employs phase detection, which can be very fast, and it works between exposures when the mirror is in the down position.

Once the camera is changed to Live View mode and the image is composed on the screen on the back of the camera, the reflex mirror is lifted and light can no longer reach the AF sensor. The majority of SLRs instead use a contrast detection AF system that uses information from the imaging sensor.

A few DSLR cameras employ a hybrid system which merges contrast detection with phase detection, which dedicated phase detection AF points on the imaging sensor. Contrast detection systems have a tendency to be significantly slower than phase detection systems.

7. SLR lightmeters take reflected readings

A few modern SLR exposure metering systems are incredibly sophisticated, but sometimes they can still be tricked by very bright or dark subjects.

This is because they measure the amount of light being reflected from a scene and typically they expect a scene to average out as a mid-tone.

If the scene is drastically lighter than mid-tone the camera may set (or suggest) exposure settings that underexpose the scene making it darker in the image than it is in reality. On the other hand, scenes that are very dark can cause overexposure.

8. Buffer capacity and card speed is important

A few cameras offer impressive continuous shooting rates which are quite handy when you’re shooting sport or action. However, these cameras can only sustain this rate for a short length of time until the buffer, the camera’s temporary storage, becomes full.

The camera clears the buffer to permit additional shots to be taken by writing the images to the removable memory card. If the card installed is cheap and slow, it will be slow to clear the buffer, so you must wait before you can take more shots.

If the camera is compatible, faster cards will allow the buffer to be cleared at a faster pace allowing more images to be taken in a sequence to lessen waiting around between shots. Buffers with larger capacity will allow more images to be stored to permit longer shooting sequences.

9. Mirror movements can cause blur

The wobble that is brought about by the mirror movements that are necessary to allow a shot to be taken are not an issue with ‘normal’ exposure times. With longer exposures, however, the wobble can cause minor blurring of the image.

By setting the camera to mirror lock-up or exposure delay mode, the issue of ‘mirror-bounce’ or ‘mirror-slap’ can be circumvented.

Once the mirror lock-up is put in place the first press of the shutter release will lift the mirror. Once the vibration has subsided, a second press will then trip the shutter.

These two shutter release actions should preferably be made with a remote release so the camera itself does not need to be touched, which can cause vibrations.

A single press of the shutter release is all that is required with exposure delay mode, the shutter fires automatically just a few seconds after the mirror is lifted.

10. The lens mount is specific to the manufacturer

One benefit of the SLR over the compact camera is that you can change lens. That said, the mount on the camera is specific to the manufacturer and you have to use compatible lenses.

Compatible lenses are available from third party manufacturers in most cases, however it’s essential to ensure that you get the correct version. Canon, Nikon and (hopefully) soon Pentax make SLRs with two different sized sensors (APS-C and full-frame) and it’s important to make certain that the lens is intended for the sensor in your camera.